Many potentially important uses of robotics involve precise positioning of 3-dimensional objects in a work station. For example, precision machining operations require placing the machining tool and workpiece in a well-defined spatial relationship. In optical fiber assembly, it is advantageous, in producing high-efficiency fiber coupling, to bring two fibers together within as little as a few microns, without damaging the fibers by actual contact. Another important application of precision-positioning robotics is in microelectronics, where sequential surface etching and metallizing operations require precise positioning of the chip substrate with respect to automated surface-treatment devices. Typically, substrate positioning to within a few microns or less is desired.
Robotics positioning is also needed in a variety of tasks, such as machine assembly of parts, robot-assisted surgical procedures, and space docking operations, where micron-range precision may not be required. Nontheless, the workpiece must be placed and oriented accurately in 3-dimensional space, typically with a precision of 0.1 to 1 millimeters.
Several methods for positioning an object in a workstation by robotic (automated) control have been proposed in the prior art. One common approach involves the use of rigid mating fixtures. Here a fixture is designed so that a robot-controlled object fits into it in a specific position, for purposes either of calibration or position control. This technique has the disadvantages, however, that the robot must avoid collision with the fixture, and generally can approach the fixture from one direction only, when attempting to mate with it.
In another positioning method, a robotic arm (effector) is programmed to return to a selected "memorized" position after picking up a workpiece. The position resolution achievable with this method is limited in part by positional variation which is related to slippage, wear, and/or variable effector response in the mechanical coupling. A second limitation is that the position of the workpiece in the robot arm will vary from one pick-up to another, and thus the actual position of the workpiece cannot be accurately related to robot arm position.
Vision systems are also used for robot positioning and calibrating robotic movement. A typical system uses a digital video camera which is focused for viewing a robot end effector carrying the object of interest. Positioning is achieved by moving the robot until a fiducial mark carried on the effector or workpiece arrives at a desired location in the video image. This approach (involving one camera only) cannot be used where three-dimensional displacements are involved. A second limitation of the method is the need for high magnification to achieve high-resolution positioning. For example, if 1-micron resolution is desired, each pixel of the video image must be less than 1 micron wide. Since typical video cameras use sensor arrays of about 350.times.350 pixels, the field of view would be less than about 1 mm.sup.2. Such a small field would be unsuitable for many of the applications indicated above.
Small displacements and surface deformations, in the approximately 1-50 micron range can be measured by holographic interometry. This method is based on the interference fringes which form when a holographic image of an object is exactly superimposed with the object itself, and the object undergoes local surface deformation (producing local interference effects) or small, rigid-body translation (producing overall interference). This method, by itself, is unsuitable for many robotics positioning problems, since interference bands are not produced unless the object and image are already in near superposition.
All of the systems mentioned above are based on positioning with respect to a fixed-position reference, such as a fixed robot base or a camera placed at a fixed position in space. Thus, the systems are not applicable to a variety of tasks involving positional interactions between two mobile robots, such as in docking procedures, and object transfer between a robot and another movable system.